Cyanopyridines are important starting materials for the production of pharmaceutical intermediates and other compounds. 3-methylpyridine (3-picoline) is an intermediate in the industrial production of nicotinic amide and nicotinic acid, which are essential vitamins of the vitamin B-complex (vitamin B3).
Methods for the production of cyanopyridines from methylpyridines are known in the art. Commonly, the cyanopyridines are oxidized in the presence of a catalyst with ammonia and oxygen. The process is referred to as “ammoxidation” or “oxidative ammonolysis”. Various catalysts are known, which comprise specific combinations of catalytic components, which can be coated on carrier materials.
WO 03/022819 discloses methods for the production of heteroaromatic nitriles by ammoxidation of the corresponding alkyl-substituted pyridines. Methods and catalysts for oxidative ammonolysis of alkylpyridines are also disclosed in WO 95/32055.
After the ammoxidation reaction, a gaseous mixture is obtained, which comprises cyanopyridine, ammonia, residual methylpyridines, side products such as pyridine, carbon dioxide, hydrogen cyanide, water and gases of the reaction stream, such as oxygen and nitrogen. It is thus necessary to isolate the cyanopyridine from this mixture. Various methods are known in the art to separate the product from the other components.
In the art, methods for isolating the cyanopyridine are known, in which the cyanopyridine is quenched and extracted with an organic solvent.
U.S. Pat. No. 2,861,299 discloses a method for obtaining cyanopyridine from a reaction product, in which the reaction product is passed through a cooling condenser, a dry ice-catcher and a glass wool filter and extracted in a collector using an inert solvent such as benzene. The extraction with benzene as a quenching agent is also disclosed in U.S. Pat. No. 3,929,811.
However, the use of organic quenching agents is disadvantageous, because organic solvents such as benzene are relatively expensive, toxic and inflammable. Further, the use of organic solvents in an industrial quenching process is problematic, because the gaseous phase is enriched in organic solvents and explosive even at room temperature. Thus the reaction product has to be cooled to low temperatures before and during quenching. The waste gas comprises high levels of organic solvent and has to be treated. Quenching methods with organic solvents are thus complicated and require a large number of process steps.
US 2008/0039632 discloses a method for quenching a gaseous reaction product comprising cyanopyridine with a predominantly non-aqueous quenching fluid. The quenching fluid comprises picoline, which is the starting compound of the ammoxidation reaction and thus can be retransferred to the reactor. However, picoline is explosive when mixed with air, and thus the process requires specific safety measures such as reducing and controlling the temperature and the oxygen content. After cooling the picoline in the separation step, it has to be transferred to the reactor and reheated, and thus the overall process requires a large amount of energy.
In order to overcome the problems associated with quenching with organic solvents, methods have been developed in the art in which the use of an organic solvent is not necessary.
CN101045706 A discloses a method in which the gaseous product obtained from the ammoxidation reaction is brought into contact with a circulatory aqueous solution in two absorption towers, in order to obtain an aqueous solution of 3-cyanopyridine. Since cyanopyridine is hydrolyzed to nicotinic acid at elevated temperatures and at a high concentration, it is necessary to control the concentration of 3-cyanopyridine in the absorption towers and in the product below 10 wt. %. Further, it is necessary to control the temperature of the circulatory aqueous solution, and thus the temperature in the two absorbent towers, to below 50° C., preferably between 15 and 30° C. When choosing such a low concentration and temperature, more than 95% of the product is recovered.
However, the concentration of the product in the final solution is relatively low and it would be desirable to obtain the product at a higher concentration. Further, the loss of 5% cyanopyridine by hydrolysis is still relatively high. The gaseous product, from which the cyanopyridine was separated, is not reused in the process. Residual picoline is lost and the process requires at least two absorption towers.